How to identify Magnetite in Garnet

To correctly identify Magnetite in Garnet (Fe3O4 in (Ca,Mg,Fe,Mn)3(Al,Fe)2(SiO4)3), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Magnetite inclusions are typically black. The host garnet can vary widely in color, including red, reddish-brown, brown, black, green, or yellow, depending on its specific chemical composition (e.g., almandine, pyrope, spessartine, grossular, andradite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Magnetite inclusions exhibit a metallic to submetallic luster. The host garnet typically has a vitreous (glassy) to resinous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The texture refers to the arrangement and size of the magnetite inclusions within the garnet. They can be disseminated, clustered, or aligned along crystallographic planes within the garnet. The host garnet itself is typically granular or massive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Magnetite inclusions often appear as small, opaque, black crystals within the garnet. They can be euhedral (octahedral, dodecahedral) if they grew unhindered, or anhedral if they were trapped or grew under confined conditions. The host garnet typically forms dodecahedral or trapezohedral crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Magnetite has no true cleavage, but exhibits parting on {111}. Garnet also lacks cleavage, displaying conchoidal to uneven fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in metamorphic rocks (e.g., schists, gneisses, amphibolites, granulites, eclogites) and some igneous rocks (e.g., granites, pegmatites, kimberlites). The presence of magnetite in garnet often indicates specific oxygen fugacity conditions during formation.

Key Facts

Hardness

Magnetite: 5.5-6.5 (Mohs). Garnet: 6.5-7.5 (Mohs).

Specific Gravity

Magnetite: 5.17-5.18. Garnet: 3.5-4.3 (varies with composition).

Crystal System

Magnetite: Isometric. Garnet: Isometric.

Color

Magnetite: Black. Garnet: Variable (red, brown, green, black, yellow).

Luster

Magnetite: Metallic to submetallic. Garnet: Vitreous to resinous.

Transparency

Magnetite: Opaque. Garnet: Transparent to translucent to opaque.

Fracture

Magnetite: Uneven to subconchoidal. Garnet: Conchoidal to uneven.

Cleavage

Magnetite: None, but parting on {111}. Garnet: None.

Composition

Magnetite: Iron oxide (Fe3O4). Garnet: Silicate mineral with variable composition (e.g., almandine Fe3Al2(SiO4)3, pyrope Mg3Al2(SiO4)3, grossular Ca3Al2(SiO4)3, andradite Ca3Fe2(SiO4)3).

Physical characteristics

• Crystal Habit: Magnetite: Octahedral, dodecahedral, granular, massive. Garnet: Dodecahedral, trapezohedral, granular, massive.
• Cleavage Type: Magnetite: None, but parting on {111}. Garnet: None.
• Fracture Type: Magnetite: Uneven to subconchoidal. Garnet: Conchoidal to uneven.
• Tenacity: Magnetite: Brittle. Garnet: Brittle.
• Luster Type: Magnetite: Metallic to submetallic. Garnet: Vitreous to resinous.

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